This invention relates to cutting elements used in earth boring bits for drilling earth formations. More specifically, this invention relates to cutting elements incorporating thermally stable polycrystalline diamond (TSP). These cutting elements are typically mounted on a bit body which is used for drilling earth formations.
A cutting element 1 (FIG. 1), such as shear cutter mounted on an earth boring bit typically has a cylindrical cemented carbide body 10, i.e. a substrate, having an end face 12 (also referred to herein as an “interface surface”). An ultra hard material layer 18, such as polycrystalline diamond (PCD) or polycrystalline cubic boron nitride (PCBN) is bonded on the interface surface forming a cutting layer. The cutting layer can have a flat or curved interface surface 14. Cutting elements are mounted on pockets 2 of an earth boring bit, such a drag bit 7, at an angle 8, as shown in FIGS. 1 and 2 and contact the earth formation 11 during drilling along edge 9 over cutting layer 18.
Generally speaking, the process for making a cutting element employs a substrate of cemented tungsten carbide where the tungsten carbide particles are cemented together with cobalt. The carbide body is placed adjacent to a layer of ultra hard material particles such as diamond or cubic boron nitride (CBN) particles within a refractory metal can, as for example a niobium can, and the combination is subjected to a nigh temperature at a high pressure where diamond or CBN is thermodynamically stabled. This results in the re-crystallization and formation of a polycrystalline diamond or polycrystalline cubic boron nitride ultra hard material layer on the cemented tungsten carbide substrate, i.e., it results in the formation of a cutting element having a cemented tungsten carbide substrate and an ultra hard material cutting layer. The ultra hard material layer may include tungsten carbide particles and/or small amounts of cobalt. Cobalt promotes the formation of polycrystalline diamond (PCD) or polycrystalline cubic boron nitride (PCBN). Cobalt may also infiltrate the diamond of CBN from the cemented tungsten carbide substrate.
The cemented tungsten carbide substrate is typically formed by placing tungsten carbide powder and a binder in a mold and then heating the binder to melting temperature causing the binder to melt and infiltrate the tungsten carbide particles fusing them together and cementing the substrate. Alternatively, the tungsten carbide powder may be cemented by the binder during the high temperature, high pressure process used to recrystallize the ultra hard material layer. In such case, the substrate material powder along with the binder are placed in the can, forming an assembly. Ultra hard material particles are provided over the substrate material to form the ultra hard material polycrystalline layer. The entire assembly is then subjected to a high temperature, high pressure process forming the cutting element having a substrate in a polycrystalline ultra hard material layer over it.
PCD ultra hard material cutting element cutting layers have low thermal stability and as such have lower abrasive resistance which is a detriment in high abrasive applications. Consequently, cutting elements are desired having improved thermal stability for use in high abrasive applications.